Clerodendrum fistulosum (Verbenaceae), an unspecific Myrmecophyte from Borneo with spontaneously opening domatia

8 Pages·1994·2 MB·English

by  Maschwitz, U

BLUMEA 39 (1994) 143-150
Clerodendrumfistulosum(Verbenaceae),
an unspecific Myrmecophyte from Borneo
with spontaneouslyopening domatia
U. Maschwitz B. Fiala & K.E.Linsenmair
Summary
ClerodendrumfistulosumBecc.is atruemyrmecophyteasitoffersnestingspaceforantsin hollow
internodes.In contrast toprevious reportsourinvestigations provedthatthese domatia open by
themselves,thus providingcavities foravariety ofdifferent antspecies. InSarawak,Malaysia, we
did not find anobligaterelationshipbetween C.fistulosum andaspecificant-partner.Forcompari-
son,studiesonherbarium material ofotherClerodendrumspecieswere carriedout:afurtherspecies,
C. deflexumfromthe MalayPeninsula andSumatrapresumably also ismyrmecophytic.
Introduction
ThegenusClerodendrumcomprises about400species, mostlypaleotropical orsub-
tropical. Somespecies havehollowinternodesandare inhabitedbyants (8 species in
Africa, 3in Asia, Schnell& Beaufort, 1966; Jolivet, 1986). Many also haveextra-
floralnectaries [Rao&Ramaya (1992)list 15species].
ClerodendrumfistulosumBecc. hasearly beendescribedas anant-plant duetoits
domatiaandespecially to havingantsinside(Beccari, 1884).(The term domatiais used
forallplant chambersthatappearto beadaptations facilitating ant nesting.) Beccari
founditin Sarawaktobeassociatedwith aspecific andobligatory partnerant,Cam-
ponotus(Colobopsis) clerodendriEmery. Ithasbeen reporteduntilrecently thatthe
searching queensofthis speciesbite entrance holesinto aswollenhollow internode
during colony foundation(e.g., Huxley, 1986; Jolivet, 1986). However,the dis-
coverer ofthis ant-plant, Beccarihimself, doubtedwhetherantswereresponsible for
theveryregular openings andspeculated thatthey were developed naturally. Obvi-
ously his opinionwas uncorrectly translatedfromtheoriginal Italianversionandthis
errorwas maintained.
InSarawak wefound aseriesoftheseplants and wereable tostudy theminthe
field.Inaddition,wewereableto cultivatefourplants inthegreenhouse inGermany
and growtwo specimens fromseed. Incontrast toprevious reports wecouldobserve
thatentrance holes develop by themselves.As themyrmecophytic statusofthis ant-
plant apparently was misunderstoodby severalauthorswebriefly reportour findings
onbiology andecology ofthisplant-ant system.
*) Zoologisches Institut,J.W. Goethe-Universitat,Siesmayerstr. 70,60054Frankfurt, Germany.
2) Lehrstuhl furTierokologieund Tropenbiologie(ZoologieIII), Biozentrum,Am Ilubland,97074
Wiirzburg,Germany.
144 BLUMEA Vol. 39, No. 1/2, 1994
Fig. 1. Upperpart ofanant-inhabited Clerodendrumfistulosumshrub. One Crematogasterantis
enteringthe domatiumentrance, another oneisclimbingintothe flowerbudpanicle.
U.Maschwitz, B.Fiala & K.E.Linsenmair: An unspecific myrmecophytefrom Borneo 145
MATERIALS AND METHODS
During astay in Sarawakin 1992fifteenspecimens ofClerodendrumfistulosum
were found in a low, undisturbedkerangas (heath) forest onpoor sandstone near
Kuching (2° 20' N, 110°25' E, 100m a.s.l.; for habitat description, see Brunig,
1965).The plants andtheirpartner ants were thoroughly checked in the field.Ant
voucherspecimens aredeposited inthecollectionofthefirstauthor.Thepresenceof
glucose ingland secretionswas tested withDextrostix(Merck).
Forobservationofdetailsofdomatiaformationandseedling growth fourplants
and two seeds, respectively, were cultivated in the greenhouse in Germany. For
comparative studiesherbariumspecimens ofClerodendrumspp. were checked atthe
Forest ResearchInstituteMalaysia (FRIM, Kepong, PeninsularMalaysia).
RESULTS
General description oftheplant
Clerodendrumfistulosum isasmallshrub.The 15plants investigated in thefield
ranged from4 to 131 cm in height and their stems were unbranched. The largest
plant, with 10internodes,carriedapanicle ofgreen-yellow budssitting ingreenred-
tipped calyces. Inthe greenhouse onespecimen, only 15cmhigh, already developed
its whiteflowers. In thefield oneplant of65 cm height bore5blackish blue drupes
with athinbright wax-cover. Whilemost plants possessed several leaf-pairs, this
specimen had shedall its leaves. Itwas colonized by Crematogaster ants. In older
plantsthedecussateleaves were situateddirectly atthetop oftheinternodes(Fig. 1).
The first eight leaves ofsmall saplings grewin spirals (greenhouse observation).
Domatia
In C.fistulosuminthe fieldtheinflatedandhollow internodesvariedfrom 1.5 to
18cm inlength (x = 7.2 cm ± 5.3, n= 28). They were club-shaped and rounded,
ratherflattenedincross section (x = 1.02 cm ± 0.13; n =9). As soonas they were
fully elongated, they becamelignified and stiff.Many ofthe internodespossessed
oneholeor apairofholeson thebroaderside oftheinternodejustbeneaththeinser-
tionoftheleafpetioles (Fig. 1).From32 internodeschecked inthefield, 11 hadtwo
open holes,14had one openhole,two fully grown internodeshad noholes atall,
and in 5 internodeswithout ants oneorbothholesapparently hadclosedagain. As
we couldobservebothin thefieldandinthe greenhouse, theholesdid not develop
synchronously. As we could concludefrom detailedobservationofthedistribution
of antswithin theinternodesand directobservation ofgrowing plants inthefield as
well as theinthegreenhouse theholes develop spontaneously through growth pro-
cesses oftheplants independently of thepresence ofants(Fig. 2).Wefoundopen
internodeswhichapparently hadneverbeeninhabitedby antsandwere stillnot colo-
nized.Allstagesofopenings werepresent. Only after development of fullyopened
holes apartofthedomatiabecame inhabited.Theholesarose as slit-likestructure
whichlaterbecamerounded[slits: x = 2.5(±0.7) x 1.7mm (±0.5); rounded holes:
diameterx = 1.2±0.5mm,n=21], Allplants inthegreenhouse developed domatia
openings inthecourse oftwoyears(8 holesin 5 plants); 11 slits didnot fullyopen.
146 BLUMEA Vol. 39, No. 1/2, 1994
A plant grownfrom seed in the green-
house developed its first inflatedinter-
nodeata heightof3cm. Atthis develop-
mentalstagetheleavesarenot yetdecus-
sate but stillspirally arranged. Thefirst
entrance slits developed more than a
yearlater.
Nectaries
The lowerleaf surfaceofC. fistulo-
sum is coveredwithnumerousnectaries
(Fig. 3).Thenectaries are c. 0.5 mm in
cross section; they develop as greenand
roundish dots on thereddish underside
ofthe young leaves. These dot glands
produced asecretionwitha glucose con-
centrationofmore than2.5 g/1 (max-
imum reading of the Dextrostix test).
Whenaccess to theplants was allowed
inthegreenhouse thenectarieswere vis-
itedby tiny antsofthegenusPlagiolepis
(Formicinae). Thefirstnectariesappear-
edonthefourth leaf(1 cmlong) on the
one-month-oldseedling (c. 1 cm high).
On leaf number7 ten nectaries were
counted.Large leavesofthegreenhouse
plantspossessed upto60nectariesmost-
ly along themainnerves.
Apparently, theinnerbaseofthebud-
carrying calyces produced extranuptial
nectar. Many workersofaCrematogas-
tersp. constantly visited thesering-like,
slightly domedstructuresaround thein-
Fig. 2.Domatiumentrance slit atthe upper end
sertionofthelongcorollae.Specialfood-
ofaninternode which didnot fully open (see
arrow; greenhouseplant;sizes are given in the bodiesasoftenoccur on myrmecophytes
text). havenotbeenobserved inC.fistulosum.
Antinhabitants
Tenofthe fifteenspecimens ofC.fistulosum studiedin thefield were inhabited
by ants. Eachofthese wasoccupied by only one ant colony. Afurtherplant hadno
live antsbuttheinternodeswere still partly filledwithdetritusand antremains.We
observed five antspecies offourgenera(ofsubfamiliesFormicinae, Dolichoderinae
and Myrmicinae) living intheinternodes:Camponotus sp. (1 colony), Iridomyrmex
sp. (2 colonies), Technomyrmex sp. (1 colony), andtwo Crematogaster spp. (each
with 2 colonies). The Camponotus sp. we found did not belong to the subgenus
Colobopsis as describedby Beccari.
U.Maschwitz, B.Fiala & K.E.Linsenmair: An unspecific myrmecophytefromBorneo 147
Fig. 3.LowerleafsurfaceofClerodendrumfistulosum.Thenumerous dotsnearthemidrib areextra-
floralnectaries.
148 BLUMEA Vol. 39, No. 1/2, 1994
We didnot findany homopterans withinthe internodesor on the surface of the
plants. In two internodessmall cockroaches were found.On some plants large
wounds were conspicuous whichhad, however, already healed. Apparently, some
animal, probably arodentor abird,had opened thedomatiato getaccess to theant
colonies.In otherareas ofSarawakwehaveseensimilarwoundsat thedomatiaof
othermyrmecophytes likeNeonaucleaand Macaranga species. In Kalimantanwe
observeda woodpecker picking intoand widening theentrance holesofMacaranga
hypoleuca. InSekinchan, WestMalaysia, wefound scars ofrodentteethonastem
ofMacaranga pruinosa.
DISCUSSION
Likeotherspecies ofthe genusClerodendrum, C. fistulosum possesses extrafloral
nectaries (Jolivet, 1986; PadmaRao &Ramayya, 1992). Apparently an unspecific
nutritional attractionofantsisa good predisposition forfurthersteps intheevolution
ofmyrmecophytism. However, as wehave discussedforthe SEAsianmyrmeco-
phytic genusMacaranga, thepresenceofextrafloralnectariesdoesnotseemto bean
essential featurein theevolutionofmyrmecophytes. Theofferofnestingspaceis of
greatimportanceforthe development ofobligate andspecific relationships withants
(Fiala & Maschwitz, 1991, 1992a, b).AlsoinC.fistulosum onlythe additionalde-
velopment ofdomatiamakesthis species a true myrmecophyte. A combinationof
boththeseant-relatedcharacterscanbefoundinmanymyrmecophytes (Jolivet, 1986).
InC.fistulosum extrafloralnectar is produced by thesapling long beforethe domatia
occur (more than six months in thegreenhouse). This orderinthe development of
ant rewards seems to be dueto plant size constraintsatfirst view. Perhaps timeof
development and also quantity and quality ofsuch rewardsshould be thoroughly
consideredalso under cost-benefit-aspects (as discussed by Davidson & Fisher,
1991). In certainMacaranga species, forinstancestrategies ofplantdefenceand ant
rewards canchange drastically during lifetimeof atree (Fialaetal„ 1994).
As was mentionedintheintroduction, C.fistulosumisreported tobeamyrmeco-
phyte associatedwithonespecific antpartner:Camponotus (Colobopsis) clerodendri.
Incontrast we did not find an obligate specific relationship between C.fistulosum
and a specific ant-partner. Theant queensare thought to getaccess to the hollow
internodesby biting holesthrough thedomatiawalls atpreformed praestomatalsites
beneaththepairedleafpetioles (Huxley, 1986; Jolivet, 1986). Also, theworkers are
reported to force theirentranceinto thehollow chambersbybiting through thethin-
walledparenchyma attheupperendof theinternodes(Blatter, 1928). Accordingto
ourobservationsalready smallplants less than 10 cm in length can produce inter-
nodialdomatiawhich soon become lignified. Our investigations proved that the
domatiaopenby growthand degeneration processes oftheplant stemitself. There-
sulting holesare veryregular inshape and develop directlybeneaththeleafpetioles.
Theyremainopenforalong timeeven whennot inhabitedby ants.This demonstrates
thattheplant offers nesting spaceforopportunistic ant species nesting inplant hol-
lowsfromtheverybeginning. As mightbeexpected manyofthesecavities are used
byavarietyofdifferentants outofseveralsubfamiliesfornesting (as describedfor
Myrmeconauclea strigosa, Rubiaceae, by Maschwitz etal., 1989).AColobopsis spe-
U.Maschwitz, B.Fiala & K.E.Linsenmair: An unspecific mynnecophytefromBorneo 149
cieswas not foundatallatin C.fistulosum atour investigation site. Such unspecific
antcolonizationmighthindercolonizationbyaspecific ant-partner(as inLeonardoxa
africana, Leguminosae, wherethespecific ant-partnerPetalomyrmexphylax, is ex-
cludedby Cataulacusmckeyi; McKey, 1984).Moreover,thesmallusually unbranch-
edshrubs donot representa good nesting opportunity fora myrmecophytic ant with
large colonies, likeaColobopsis sp.
Inthesamehabitatwefoundanothermyrmecophyte, viz.Macaranga caladiifolia,
containing amongother antspecies two Colobopsis spp.indicating thatarborealants
of that subgenus were commonthere.However,the ant faunacomposition ofM. ca-
ladiifolia was neverthelessrathersimilartothatofC.fistulosum, Crematogasterand
Camponotus beingthemostabundantants.
FromSEAsiatwo moretrue myrmecophytic (i.e. domatia-bearing) Clerodendrum
taxa arereported: C.phyllomega Steud.var. myrmecophilum (Ridley) Moldenkeand
C. breviflorum Ridley, both small shrubs.Theformerspecies occurs inBorneoand
Sumatra, thelatterisendemic totheMalay Peninsula(Ng, 1978).
Jolivet(1985) reported ClerodendrumfallaxLindley fromJavaas inhabitedby a
numberof differentantspecies whichseemtointrudeby leafscars. IntheCape Verde
Archipelago, wherethisspecies was introduced, it isalso inhabitedby ants.In Joli-
vet's opinioninthisspecies theevolutiontowardsmyrmecophytism is less perfectas
intheabovementionedSoutheastAsian species sincetheinternodesare not inflated.
Wehadnoopportunity to see specimens oftheselattermentionedspecies. How-
ever, inherbariumspecimens ofotherClerodendrumswe didnot detectany domatia
onthefollowing species: C. hispidum Hend.,C. inerme(L.) Gaertn.,C. nutans Jack,
C. paniculatum L„ C. ridleyi King & Gamble, C. serratum Spreng., C. umbratile
King & Gamble, C. villosumBlume.Onefurtherspecies, however, displays myrme-
cophytic characters: C. deflexum Wall.var. bracteatumRidley. Itis commonthrough-
out the Malay Peninsulaand Sumatra (Ng, 1978). It possesses hollowand partly
inflatedinternodes.Theopenings, however,are not paired and not situated atafixed
pointoftheinternodeas in C.fistulosum. Wesupposethatalso this species is atrue
myrmecophyte associatedwithunspecific ants nesting inliving wood.
ACKNOWLEDGEMENTS
This study was supportedby the Deutsche Forschungsgemeinschaft.Wearegratefultoananony-
mousrefereeforvaluable suggestionsand helpfulcorrectionsofthemanuscript.Wethank A.Schel-
lerich forhelpwith identifyingthe ants.
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